My classic STI has got a bad missfire in the mornings if it has been raining or damp. It runs fine when it has warmed up. I have already changed the plugs and Temp. sensor because I miss diagnosed it as a cold start problem. It ran fine all the time the ouside temperature was freezing and we had snow, now it has turned to rain the problem has returned. Fault codes are showing none???

 

what year is it as that would help?

 

It's a 95 STI 555.

 

check for cracked coil packs, and if it has an induction kit make sure the bonnet vent that side has the cover on it inside or the maf will be getting wet causing the issues

 

It does have an induction kit. Bonnet vent cover is in place, is it worth making the MAF watertight (plastic bag and gaffer tape Trick) just to check? And to check the coil packs, do I just take them off and visually inspect them for cracks?

 

Pull the rubber plugs off the sparkplug ends of the coilpacks to check them

When undoing or tightening the coilpack 12mm bolt, you need to support the coilpack to stop it from twisting - This can cause them to crack.

 

This, but he didn't mean the top of the sensor getting wet, he meant the filament inside.
Basically water on the air filter getting sucked in cools the wire and makes it read wrong.

You need to keep the water off the airfiler, not the actual sensor.

 

Thanks for all your advice, I seem to have solved the problem (fingers crossed) by taking off the MAF sensor and cleaning it out with some carb cleaner. The last couple of times it has been wet, it has started and run fine and is generally running much sweeter.